Known examples of nuclear reactors in nuclear power generation systems include an indirect-cycle reactor for generating electricity by rotating a turbine with radiation-uncontaminated water vapor. Such a nuclear reactor has steam generators and heat exchangers between the primary and secondary cooling systems.
A loop-type fast reactor intended for the construction of a large power generation system transfers the heat in the primary sodium system (primary cooling system) generated by cooling the reactor core to the secondary sodium system (secondary cooling system) through intermediate heat exchangers, and also transfers the heat in the secondary sodium system to a water-steam system through an evaporator and a superheater. Similarly, a pool-type fast breeder reactor with a larger reactor vessel containing the pump of the primary sodium system and the intermediate heat exchangers transfers the heat in the primary sodium system to the secondary sodium system through the intermediate heat exchangers, and also transfers the heat in the secondary sodium system to the water-steam system through the steam generators.
A nuclear reactor in such a large power generation system includes a reactor core having the assemblies of a large number of fuel rods, each of which is a cladding tube containing fuel pellets made of metallic oxides including uranium-235 or plutonium-239, which has low heat transfer characteristics. The reactor core of a large nuclear reactor has about 200 fuel assemblies, each of which is a bundle of several tens of fuel rods, and control rods for controlling the reaction rate of the fuel are positioned between these fuel assemblies. In such a large nuclear reactor having control rods, a loss of the control of rod functionality due to, for example, trouble in the mechanism of controlling the positions of the control rods may cause a runaway of nuclear reactions in the reactor core.
Nuclear reactors other than fast breeder reactors, such as a pressurized light-water reactor, transfer the heat in the primary coolant generated by cooling the reactor core to the water-steam system through the steam generators. Such a nuclear reactor also has control rods positioned between the fuel assemblies contained in the reactor for controlling the reaction rate in the reactor core.